In electric power stations, a boiler supplies heat to the fluid of a circuit in a closed loop, this heated fluid flowing then in a steam generator so as to give up its heat to the water which is converted into steam, the steam being then sent to the turbines of the power station.
Some nuclear power stations, in particular of the fast neutron type, employ as fluid for transporting heat from the boiler to the steam generator, liquid sodium. In this case, the steam generator comprises a heat exchanger whose primary circuit encloses the liquid sodium and whose secondary circuit encloses water converted into steam.
These steam generators comprise a cylindrical shell which has a vertical axis and is closed by crowned or dished end walls in which is disposed the tube bundle. This bundle, when it is formed by tubes wound helically, occupies only a part of the cross-section of the internal volume of the shell and extends only in a part of the height of the shell. Formed above the bundle is a region in which at least one heated liquid sodium supply pipe opens, whereas in the lower part of the steam generator there is provided a cooled liquid sodium outlet pipe.
The liquid sodium flows, between its inlet and outlet in the shell of the steam generator, in contact with the outer surface of the tubes of the tube bundle in which flows water which is converted into steam by heat exchange with the liquid sodium.
The central part of the inner volume of the shell is occupied by a cylindrical central body closed in its upper part, coaxial with the shell of the steam generator, from the lower part of the latter to the upper region in which the liquid sodium arrives. The tube bundle occupies the peripheral part of the internal volume of the shell around the central body throughout the height of the latter, except for its lower part in which openings permit the outlet of the liquid sodium after it has passed through the bundle without creating disturbances in the flow in the region of the outlet loops of the tubes of the bundle.
The supply and flow of the sodium in the steam generator are regulated in such manner that the upper level of this sodium in the shell of the generator is located a little above the inlet of the sodium in the upper part of the shell. The space between the upper level of sodium and the upper end wall of the generator is filled with a neutral gas such as argon.
In such steam generators, precautions must be taken to avoid any contact between the liquid sodium of the primary circuit and the water of the secondary circuit. Indeed, it is known that the mixture of sodium and water at high temperature produces very violent chemical reactions with the emanation of gas and a sudden increase in the pressure prevailing in the liquid sodium. These accidental sudden reactions, which may be likened to an explosion inside the generator, can result in partial deterioration of certain components, but also, owing to the propagation of the pressure wave in the piping of the secondary circuit, can damage the intermediate exchangers or circulation pumps or any installation located in this secondary circuit. It is therefore necessary to encourage the propagation of the pressure waves toward a free level of the liquid sodium where these waves are damped.
Up to the present time, in the event of a sodium-water reaction, the pressure wave which travels upwardly receives a considerable damping or absorption on the upper free surface of the sodium, on the other hand, that which travels downwardly does not encounter such a free surface.